Submitted to ANS/NT (2021), LA-UR-21-21007 Thirty Minutes Before the Dawn—Trinity Alan B. Carr* Los Alamos National Laboratory Los Alamos, NM 87545 The Trinity test of July 16, 1945, marked the scientific apex of the Manhattan Project. Often recognized as the symbolic birth of the nuclear age, Trinity’s multifaceted legacy remains just as captivating and complex today as it did 75 years ago. This paper examines why the test was necessary from a technical standpoint, shows how Los Alamos scientists planned the event, and explores the physical and emotional aftermaths of Trinity. The author also uses rarely accessed original records to reconstruct the story of Trinity’s health hazards, as seen through the eyes of radiation technicians and medical doctors as events unfolded. Trinity was conducted as the Potsdam Conference began, weeks after the collapse of Nazi Germany. It was considered necessary to let President Harry S. Truman know whether the United States possessed a nuclear capability ahead of his negotiations with Joseph Stalin, the Soviet premier. The author examines the competing priorities that drove the timetable for the test: international politics, security, and safety. Three weeks after Trinity, a gun-assembled enriched-uranium bomb called Little Boy was used against the Japanese city of Hiroshima. Three days later, Fat Man, a weaponized version of the imploding Trinity device, was dropped on Nagasaki. The author briefly examines these strikes and what impact they may have had on the Japanese surrender. The paper concludes by examining the legacy of the Trinity test 75 years into the age it helped usher in. Keywords: Trinity, Los Alamos, Oppenheimer, fallout I. One of the Great Events of History weapons developed during the war? When the work at Los Alamos began, the most promising path to success Seventy-five years ago, Los Alamos scientists secretly appeared to be constructing gun-type weapons because, conducted the world’s first nuclear weapons test. The from an engineering standpoint, gun assembly seemed story of this historic event is well known; it has been less complex and far more certain than proposed shared many times, by many people, over the decades. alternatives. In a gun-assembled nuclear weapon, a But this test, dubbed “Trinity” by Los Alamos Director J. subcritical mass of fissile material is fired at another Robert Oppenheimer, did not happen in a vacuum. As the subcritical fissile mass to produce a nuclear detonation. first day of the nuclear age dawned in New Mexico, The plutonium gun weapon was given the name Thin fighting continued throughout Japan’s disintegrating Man; Little Boy was its enriched uranium counterpart. empire in places such as Borneo, Burma, China, and the But in the spring of 1944, experiments performed by Philippines. In the coming weeks, Stalin’s armies would future Nobel laureate Emilio Segrè began to cast doubt on bring the war to the Japanese in Manchuria and Sakhalin. the viability of Thin Man: such a device might The large cities of Japan endured heavy bombing predetonate because of spontaneous fission in the isotope throughout this period, while kamikazes desperately tried 240Pu. That July, Segrè’s troubling results were to break the ever tightening Allied blockade. But when confirmed: Thin Man would detonate before it was fully Oppenheimer’s fearsome creation detonated in the New assembled.1 The demise of Thin Man is where the story Mexican desert, there was awe-inspiring silence in the of Trinity begins. immediate aftermath. Of course, it would not last: the fleeting serenity would be broken after several moments II. All Possible Priority by the passage of a violent shock wave. Soon, that same At an administrative board meeting on the morning of elemental force would break the morning in Japan, as July 20, 1944, held just hours after Hitler narrowly well, and, in doing so help break the Japanese escaped an assassination attempt in distant East Prussia, government’s will to continue the war. The course of Oppenheimer directed, “All possible priority should be history rarely changes dramatically in just an instant, but given to the implosion program. At the same time, that’s exactly what happened the morning of July 16, nothing essential to the 25 [code for 235U] gun should be 1945. left undone.”2 In an imploding weapon, a sphere of fissile But why perform a test in the first place? And, more material is surrounded by high explosives (HE); when the fundamentally, why were two entirely different types of HE detonates, the blast wave compresses the fissile core * E-mail: [email protected] Submitted to ANS/NT (2021), LA-UR-21-21007 to supercriticality, thus producing a nuclear detonation overcome many daunting technical challenges quickly. In (see Brown and Borovina3 and Moore,4 this issue). The late 1944 and into the spring of 1945, as plutonium and implosion concept was more complex than a gun, but highly enriched uranium were becoming available in such a design would overcome the predetonation problem greater amounts,6 hundreds of experiments were and require less fissile material. Meanwhile, progress was performed to try to better understand the hydrodynamics being made to determine the critical masses and hence the of implosion. Scientists struggled to develop a reliable amounts of special nuclear material needed (see detonator and a circuit for firing dozens of them Chadwick,5 Hutchinson et al.,6 and Kimpland et al.,7 this simultaneously. The bomb would rely on thousands of issue). Just two weeks later, Oppenheimer reorganized pounds of HE to drive the implosion; the large blocks of the Laboratory to make the implosion concept a reality. HE, which fit together like a spherical, three-dimensional Two new divisions were created to develop the “gadget,” jigsaw puzzle, would need to be precisely shaped and as the implosion bomb would become known. The first, skillfully cast. As work progressed, confidence increased. the Weapons Physics, or Gadget, Division (G) was led by See Martz et al.15 and Crockett and Freibert,16 this issue, Robert F. Bacher, formerly head of the Physics Division. on the remarkable properties of plutonium that needed to George Kistiakowsky, a Ukrainian-born veteran of the be understood. The design innovations in the Theoretical Russian Civil War, would lead the Explosives Division Division that led to the “Christy gadget,” a spherical solid (X). Both divisions were formally established on August plutonium core, are described in this issue by Chadwick 14, 1944: the Japanese emperor would announce the and Chadwick.17 termination of hostilities exactly one year later, thanks in At no point, however, were most scientists confident part to the work of these new organizations.8 enough to put an implosion bomb into combat without a The Theoretical Division under Hans Bethe’s full-scale test first. Kenneth Bainbridge, the Harvard leadership played a central role in advancing the basic physicist whom Oppenheimer would soon entrust to serve science studied during the Manhattan Project. These as test director, offers two reasons. First, “A test of the include shock hydrodynamics (Morgan and Archer,9 this atomic bomb was considered essential by the Director and issue) and neutronics (Sood et al.,10 this issue). Bethe and most of the group and division leaders of the Laboratory Feynman, both future Nobel laureates, developed an because of the enormous step from the differential and important equation for predicting the expected nuclear integral experiments, and theory, to a practical gadget.” fission efficiency, as described by Lestone.11 Computing And, “No one was content that the first trial of a Fat Man using both “human computers” and IBM punched-card (F. M.) gadget should be over enemy territory, where, if machines enabled these Theoretical Division efforts, too, the gadget failed, the surprise factor would be lost and the as described by Lewis12 and Archer.13 enemy might be presented with a large amount of active It is well known that Little Boy entered combat without material in recoverable form.”18 When the weapon a full-scale test, but there is more to the story. Every entered combat, there could be absolutely no doubt it component of the gun weapon was rigorously tested at would work. The implosion bomb’s complex and Los Alamos. For instance, nuclear criticality experiments revolutionary design demanded a test. confirmed that the Little Boy design was reliable: the odds of a malfunction were astronomically small. So why III. Planning the Unprecedented even pursue an imploding bomb if the Laboratory already Over the years, many have conjectured where the name had a very promising design? Though Little Boy was Trinity came from. In fact, it was Oppenheimer who reliable, the design suffered from a significant flaw—it named Trinity. In October 1962, as General Leslie R. was terribly inefficient. The challenges of enriching Groves, commander of the Manhattan Engineer District, uranium meant that there was not enough material to was preparing his memoir, he wrote to his former rapidly replicate combat units. This flaw was noted in a subordinate to inquire about the test’s legendary name. A Laboratory memo by future Nobel laureate Norman few days later, in the midst of the Cuban Missile Crisis, Ramsey: “The frequency of availability of active units Oppenheimer responded, “I did suggest it,” but will be sufficiently low for some time that their military continued, “Why I chose the name is not clear, but I know effectiveness will probably be relatively small.”14 In what thoughts were in my mind.” The former Los Alamos short, Little Boy was little more than a one-off gimmick, director had been reading the poetry of John Donne at the not an easily reproduced weapon.